Reinforced concrete (RC) is a composite material created by embedding steel reinforcement bars (rebar) within a concrete matrix. Concrete provides excellent compressive strength and acts as a thermal insulator for the steel, which offers tensile strength. Although concrete is non-combustible, exposure to extreme fire temperatures initiates complex physical and chemical reactions. Understanding these reactions is necessary to assess the fire performance and residual structural capacity of the structure.
How Concrete Changes Under Heat Exposure
The concrete matrix degrades when exposed to fire, starting with the chemical transformation of the cement paste. Above 300°C (570°F), chemically bound water is expelled from the calcium-silicate-hydrate (C-S-H) gel (dehydration). This decomposition causes the paste to shrink and significantly lose compressive strength and stiffness.
A second chemical change occurs between 400°C and 550°C when calcium hydroxide (portlandite) decomposes, further reducing structural capacity. This thermal degradation creates micro-cracking and increases internal porosity, lowering the concrete’s insulation quality. Pinkish discoloration often indicates this temperature threshold has been surpassed.
The most immediate failure mechanism is spalling, the chipping and flaking of the concrete surface. Spalling is driven by the build-up of internal pore pressure as trapped moisture vaporizes into steam. Because concrete has low permeability, the steam cannot escape quickly, causing internal pressure to exceed the concrete’s tensile strength.
Differential thermal expansion also contributes to spalling and cracking, as aggregates and cement paste expand at different rates. The combined effect of pore pressure and thermal stress causes pieces of concrete to break away. This exposes the underlying steel reinforcement directly to the heat, accelerating structural degradation.
The Effect of High Temperatures on Reinforcing Steel
The embedded steel reinforcement (rebar) experiences a significant reduction in mechanical properties as its temperature rises. Steel’s yield strength starts to degrade noticeably above 500°C. Above 700°C, the steel loses a substantial portion of its design strength, becoming prone to deformation under structural load.
The stiffness (Young’s modulus) also decreases, though this drop is less pronounced than the loss of strength up to 600°C. This reduction means the structural element deflects more easily under load.
A complication arises from the difference in the coefficient of thermal expansion between steel and concrete. Steel rebar expands more than the concrete mass when heated. This differential expansion creates internal stresses at the interface, promoting cracking and weakening the critical bond.
Loss of Composite Action and Structural Integrity
Failure results from the breakdown of composite action, where materials no longer work together effectively. This relies on a strong bond that allows the concrete to protect the steel and transfer forces. As the concrete cracks and the steel expands, the bond strength rapidly deteriorates.
Bond strength can be reduced by nearly half once the interface temperature reaches 300°C. This loss prevents the structural element from effectively distributing loads between the compression zone (concrete) and the tension zone (steel), resulting in a rapid decline in load-bearing capacity.
When spalling removes the protective concrete cover, the steel reinforcement is directly exposed to high fire temperatures, accelerating strength loss. The combination of weakened steel and compromised concrete leads to excessive structural deflection under load. This progressive loss of strength and stiffness eventually results in structural collapse.
Key Factors Determining Fire Performance
The severity of reinforced concrete’s reaction to fire depends heavily on its design and composition. The depth of the concrete cover (distance between the rebar and the exposed surface) is the primary variable controlling the steel’s temperature rise. A greater depth provides thicker insulation, delaying the rebar from reaching its critical 500°C threshold.
The initial moisture content also influences the risk of spalling. Concrete with higher trapped moisture, such as low-permeability mixes, is more susceptible to explosive spalling. This occurs because moisture vaporizes into steam faster than it can escape, resulting in higher internal pressure.
The type of aggregate used affects thermal stability. Concrete made with carbonate aggregates (e.g., limestone) performs better than concrete containing siliceous aggregates (e.g., quartz). Carbonate aggregates have a higher specific heat capacity and are more stable, reducing differential thermal expansion and cracking.